In packet-based wireless communication systems, a transmitted packet may be received with a large range of signal strengths, that is, a wide dynamic range. For example, in an 802.11 system, there may be as much as a 100 dB difference in signal strength between packets received at receiver A sent from transmitter B versus a packet received by receiver A sent from transmitter C. Factors accounting for this variation include path loss and fading characteristics of a RF propagation channel, for example. Path loss may include attenuation losses incurred due to the distance existing between a transmitter and a receiver. Fading characteristics of the RF propagation channel may include multipath interference destructively combining to reduce the strength of the signal received at the receiver. A well-designed communication transceiver must perform reliably given these impairments that are characteristic of wireless media. In this regard, a goal of a well-designed communication transceiver is to mitigate these characteristic impairments. In order to achieve this goal, a practical receiver may make use of automatic gain control (AGC). Automatic gain control can be described as an algorithm that may be adapted to automatically adjust signal size in order to maximize some parameter.
FIG. 1 is a block diagram of a conventional receiver system that utilizes gain control. Referring to FIG. 1, the conventional receiver comprises a mixer 102, a gain block 104, analog-to-digital converter (ADC) 106 and gain control block 108. The conventional receiver may be part of a packet-based wireless system, which is adapted to receive a signal that is transmitted at a particular carrier frequency.
In operation, the mixer 102 receives an input received signal and mixes the received signal with a tuning frequency to generate a baseband signal. The gain block 104 applies an initial gain Ginitial to the baseband signal, and the AGC algorithm will apply a final gain output gain Gfinal to the data portion of the packet. The analog to digital converter (ADC) 106 converts the analog signal to digital samples, which are subsequently processed.
A good AGC algorithm that may be implemented in the gain block 108, is adapted to choose or provide a final gain value Gfinal dB to apply to the data portion of the packet such that the signal to quantization noise ratio out of the ADC is maximized. Additionally, the final gain value Gfinal dB is chosen so that it is not too large as to cause an overflow to occur at the ADC during reception of the packet. The first criterion maximizes the signal to quantization noise ratio (SQNR) for the packet, and the second criterion prevents the packet from almost certainly being received with errors due to signal distortion. A well-designed gain block 108 is configured to execute an AGC algorithm that will accomplish this task.
Referring to FIG. 1, in L1 and L2 represents the limits of the ADC 106. In case 1, Gfinal is too small and the resulting analog signal, which is an input to the ADC 106, does not optimally utilize the limits L1 and L2 of the ADC 106. Accordingly, the AGC algorithm would have made a poor decision or choice. In case 2, Gfinal is too large and the resulting analog signal, which is an input to the ADC 106, does not optimally utilize the limits L1 and L2 since these limits of the ADC 106 are exceeded. Since the limits L1, L2 of the ADC are exceeded, clipping of the signal occurs. Accordingly, the AGC algorithm would have made a poor decision or choice. In case 3, Gfinal is ideal and the resulting analog signal, which is an input to the ADC 106, optimally utilizes the limits L1 and L2 of the ADC 106. In this case, no clipping of the analog signal occurs. Accordingly, the AGC algorithm would have made an ideal decision or choice.
For 802.11 orthogonal frequency division multiplexing (OFDM) systems, the gain Gfinal is calculated and applied during the preamble portion of the packet. The preamble of the packet is relatively short in time compared to the overall packet length, and corrections for other system impairments such as frequency offset may also need to be calculated during this portion of the transmission. Thus, the amount of time needed to determine the proper gain setting for the received packet needs to be kept small. For a practical 802.11a/g orthogonal frequency division multiplexing system, this means it is likely at most one intermediate gain setting Gintermediate is allowed during the preamble to determine the final gain Gfinal.
FIG. 2 is a diagram illustrating the application of gain to a packet. Referring to FIG. 2, there is shown a packet 200 having a preamble portion 202 and a data portion 204. The leftmost portion of the packet 200 is the demarcation of the start of packet (SOP) and the rightmost portion of the packet 200 is the demarcation of the end-of-packet EOP. The gain Gfinal is applied at reference A, which occurs during the preamble portion 202 of the packet 200. In this case, Gfinal is greater than Ginitial (Gfinal>Ginitial).
FIG. 3 is a diagram illustrating the application of gain to a packet. Referring to FIG. 3, there is shown a packet 300 having a preamble portion 302 and a data portion 304. The leftmost portion of the packet 300 is the demarcation of the start of packet (SOP) and the rightmost portion of the packet 300 is the demarcation of the end-of-packet EOP. A gain Ginitial is in effect at the start-of-packet (SOP) where clipping is occurring. A gain Gintermediate is applied at reference B where no clipping occurs but the signal is too small. A gain Gfinal is applied at reference C where no clipping occurs and the signal is ideal. In this case, Ginitial, Gintermediate and Gfinal are applied during the preamble.
In order for a receiver to detect small receiver signal input, the initial front-end gain Ginitial must necessarily be set to a large value. However, if the incoming signal is in fact large, the signal level at the output of the ADC will be clipped, making it difficult to determine the received signal power. That is, if a received signal power of X dBm is enough to cause a clip at the ADC, then all received signal powers greater than X dBm also cause a clip.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.